Heat-resistant honeycombs made of ceramics such as cordierites have conventionally been used as DPFs. However, the ceramic honeycombs are easily broken by vibration or thermal shock. Further, because ceramics have low thermal conductivity, heat spots are locally provided by the combustion of carbon-based particulates trapped in the filter, resulting in cracking and erosion of the ceramic filter. Thus, DPFs made of metals, which are higher in strength and thermal conductivity than ceramics, have been proposed.
For example, a porous metal filter having a three-dimensional network structure is proposed by JP 5-312017 A. This filter is excellent in crack resistance and erosion resistance, and the structure thereof can be more simplified as compared with the honeycombs.
Although the above porous metal body having a three-dimensional network structure is advantageous in resistance to cracking caused by thermal shock and to erosion by a melt, it is poor in a function of capturing particulates. This is because the known porous metal body having a three-dimensional network structure has a thin skeleton with a small surface area, resulting in small chances of contact of particulates with the filter body. Further, because the filter body has a smooth surface, particulates are not so stably captured by the filter body that particulates accumulated on the filter body to some extent easily peel off. It is thus considered that porous metal bodies are low in a capturing rate of particulates, failing to sufficiently capture particulates emitted from engines, thereby releasing a larger proportion of the particulates to the air. To increase a capturing rate, the porous metal bodies need to be made larger and thicker, resulting in too large DPFs for use in automobiles.